The invention relates to an apparatus for disengaging a normally-engaged, spring-loaded clutch to assist a vehicle operator in actuating a lever, linkage or similar mechanism to disengage the clutch against the action of the spring.
Friction clutches are often used for engaging and disengaging a vehicle engine from its transmission, and such clutches are usually of the spring-loaded, normally-engaged type in which a compression spring biases a driven clutch member into engagement with the drive clutch member. These clutch members are capable of transmitting substantial torque loads particularly when used on heavy vehicles such as trucks or tractors. Since the torque on these vehicles is quite high, the springs employed for engaging the clutch must have a high compression strength to obtain sufficient pressure between the drive and the driven members to prevent slippage. It is desirable then that the compression springs be a substantially greater force than any opposing force or forces to create a more effective clutch engagement for transmitting the torque.
However, there are countervailing considerations in connection with the magnitude of the spring force employed for biasing the clutch members together. To disengage a spring-loaded clutch the biasing action of the engaging springs must be overcome, and thus the magnitude of any spring force employed for this purpose should be limited to near the force it takes to disengage the clutch mechanism. For a disengaging operation the operator usually moves a foot pedal or hand lever connected through a linkage to the clutch members to overcome the action of the spring. Upon release of this lever the clutch reverts to its normal engaged position under the action of the springs. When an operator moves the clutch-operating lever to disengage the clutch by overcoming the force of the engaging springs he must hold the clutch lever in this disengaged position during the change in gear shift ratio and until the clutch is to be reengaged. This operation can be performed a multitude of times particularly in driving in city traffic or other similar traffic where many stops and starts are required. As a result an operator expends a considerable time and effort in physically overcoming the high compression of the clutch springs.
To relieve the operator of this burden many devices have been employed to allow the use of engaging springs of greater force and still permit the operator to easily and conveniently overcome the biasing action of such springs. These devices have been called assisting or boosting devices and are characterized by, for example, an over-center spring mechanism which is a tension spring having one end pivotally attached to a pivotally-mounted, actuating lever and having its other end pivotally attached to a fixed support. A specific example of an over-center spring mechanism is shown in U.S. Pat. No. 1,927,643. In this patent the spring is adapted to operate over the center line which lies through the rotational axis of the lever and the pivotal attachment of the springs to the support where the spring urges the lever to rotate in a direction to disengage the clutch when it is on one side of the center line and in opposition to the rotational urging force on the lever caused by the clutch engaging springs. The force produced in this manner on the lever is the product of tension force developed by the spring multiplied by its effective moment arm which is the length of the line perpendicular to the rotational axis of the lever and perpendicular to the direction of the tension force. Thus, the spring becomes increasingly more effective as the lever is moved further and further towards its clutch disengaged position since the effective moment arm becomes greater.
A problem with these devices is that a substantial lever movement is required before the spring reaches the position where it is effective in substantially assisting the operator in releasing the clutch. Another problem with these types of springs lies in controlling the amount of biasing caused by the increased moment during actuation of the lever. It is undesirable that the force rotating the lever should ever reach a value greater than the biasing action of the clutch engaging springs since the clutch will not return to its original position when the lever is deactivated. As a result, care must be taken in effecting the relationship between the spring and the position of the lever to assure that the biasing force never reaches a value greater than that of the clutch engaging springs.
On approach in solving this problem has been a spring and piston assisting device as shown in the U.S. Pat. No. 3,187,867 to Sink which relates to an assist device used to rotate a clutch actuating lever to disengage a normally-engaged, spring-loaded clutch. The assist device includes a cylindrical casing with an actuating rod for reciprocal movement within the casing. An exposed portion of the rod extending from one end of the casing is engaged with a linkage for actuating the clutch, and the cylinder casing is rotatably attached at another end to a stationary support. A spring located at one end of the cylinder within the casing engages a bushing which in turn is pressed against a flange of the casing. The other end of the spring engages a bushing which in the normal position engages balls resting in peripherally-spaced grooves formed in the casing. The rod also includes complementary grooves about its outer surface which in a normal position are laterally displaced from the grooves in the cylinder. However, upon actuation of the clutch mechanism the rod or shaft will be pressed inwardly relative to the cylinder such that the grooves in the rod will eventually register with the grooves in the cylinder casing. At this registered position the spring acting on the bushing transmits force through the balls and shaft thereby imparting an assist force to the operator actuating the system.
This system also suffers from deficiencies in that the balls and grooves for transmitting the force require close tolerances to insure that the spring force is properly transmitted to the shaft for providing the assistance needed to actuate the clutch.
The assist mechanism of the present invention overcomes many of the problems of complexity, expense and unreliability which have characterized assist mechanisms of the past. In one embodiment of the invention the mechanism includes a housing which is fixed relative to a rotatable clutch release shaft which is in turn keyed to a yoke for driving a clutch member away from an engaged position. Within the housing there is a sector splined to the clutch release shaft to produce rotation of the shaft and ultimately rotation of the yoke. Engaged with this sector is a rack which provides input for operator pedal effort and is spring-loaded to produce the assist force to the input. A portion of the rack extends beyond the housing for engagement with the linkage to a pedal within the cab or other actuating mechanism to move the clutch assist mechanism. A "free-travel" mechanism within the housing provides for a degree of unassisted relative movement by the rack with respect to the housing when the rack is actuated through the clutch-actuating linkage. Upon traversing the "free-travel" extent of rack movement, the rack will overcome the "free-travel" mechanism and permit the spring within the housing to assist in driving the rack relative to the housing toward a clutch-disengaged position. This rack movement is transmitted into rotational movement through the sector to complete the disengagement of the clutch.
In one embodiment of the invention the "free-travel" is achieved through a piston-cylinder arrangement where the assist spring is engaged with the piston for providing the assisting force during movement toward a clutch disengaged position. The piston, however, is constrained from movement by interaction of balls with grooves in the cylinder walls until the piston is released by action of a rack after "free-travel" movement is completed. During the "free-travel" portion of the rack traverse, the rack movement is relative to the stationary piston. During movement toward the clutch disengaged position after an initial "free" travel, grooves in the rack register with the balls. In this position the balls engage rack recesses to provide a means for permitting the assist spring to operate on the rack.
In another embodiment of the invention the "free-travel" is achieved through a piston-cylinder arrangement integrated with a detent mechanism to provide for a "free-travel" movement before the assist spring is actuated. The piston is constrained from movement by the interaction of a spring-loaded poppet ball until release by action of the rack after "free-travel" portion of rack movement has been traversed. On a portion of the rack entending through the piston, blocking members are spaced apart on either side of the piston a distance equivalent to the length of "free-travel". During movement toward a clutch disengaged position one of the blocking members can engage the piston after an initial "free-travel" and release the detent mechanism for permitting the assist spring to operate on the rack in a manner similar to that of the first embodiment discussed above.
The provision of unassisted initial or "free-travel", allows the operator to "feel" the necessary "free-travel" between the clutch release yoke and the release bearing. This is required to determine if the clutch needs adjusting, since as the driven members wear, the release bearing moves toward the fly wheel. Thus, loss of free travel is indicative of clutch wear and serves to readily determine means for assuring that clutch is properly adjusted. In cases of clutches with internal self-adjusting mechanisms, maintenance of free travel assures that the self-adjustor is working properly.
The invention provides the ability to tailor the device, maximize control and avoid unreasonably high pedal efforts or excessive, larger pedal travel which produces operator fatigue and discomfort. The timing of the application and the amount of the bias is a function of, among other factors, the positioning of the rack "free-travel" mechanism and housing stop mechanisms, the size of the rack and sector, and the amount of force and rate of the assist spring. Another advantage which results from the invention is the diminishing of the assist force of the spring during rack movement as disengagement is continued. Some popular types of heavy duty, pull-type clutches exhibit release bearing load curves which rise to a maximum value during initial release, and thereafter diminish until full release is achieved. In the present invention as the assist spring extends with rack translation, the amount of assist force diminshes which may produce a nearly constant, comfortable pedal effort from engaged to release or disengaged positions. On clutches having increasing bearing loads with increasing release travel, increasing pedal efforts could be reduced. Staging of stiff springs would also be possible to overcome the increased pedal effort required.
In the case of full power hydraulic and pneumatic arrangements such "feel" as discussed above in connection with the invention is difficult to obtain and often requires the addition of detenting devices within the control valving system. However, this is to be distinguished from a full hydraulic arrangement where the operator's input energy to the pedal creates the energy to displace the fluid and release the clutch as opposed to an engine driven pump. In the former system there is little loss of "feel". Rather, the amount of input effort is proportional to the amount of resistance at the output.
Other advantages are afforded by the self containment of the mechanism of the present invention. Not only is clutch disengagement facilitated by eliminating the need for substantial anchorage and securing means associated with the over-the-center-spring designs, but the unit of the present invention also safely contains the spring mechanism to avoid injury. The unit is sealed and lubricated, protecting the more critical components against a rather harsh track and road environment. In the event the assist spring should fail the mechanism would fail safe since the clutch could still be released, although with substantially more effort, and secondary damage would be minimal. The use of a rack and sector to gain additional mechanical advantage and the use of a coiled spring configuration compacts the design of the invention, and allows the use of a high reliability spring with excellent fatigue strength. The combination of external pressure and high stress has often been a problem with leaf spring-assist mechanisms.
Other advantages of the invention will become more apparent from a discussion of the preferred embodiments which follows.